The present invention relates generally to power control for high speed packet data access in mobile communication networks.
The current Universal Mobile Telecommunication System (UMTS) standard implements uplink power control to control the transmit power of mobile terminals on uplink channels. The uplink transmit power control procedure simultaneously controls the power of a Dedicated Physical Control Channel (DPCCH) and its corresponding Dedicated Physical Data Channels (DPDCHs), High Speed Dedicated Physical Control Channel (HS-DPCCH), and Enhanced Dedicated Physical Control and Data Channels (E-DPCCH and E-DPDCH). The power control procedure in UMTS includes inner-loop power control and outer-loop power control. Inner-loop power control compares a signal-to-interference ratio (SIR) of a received signal from a mobile terminal with an SIR target to generate transmit power control (TPC) commands to instruct the mobile terminal to either increase or decrease its transmit power. Outer-loop power control adjusts the SIR target to obtain a certain quality of service (QoS). For example, adjustment of the SIR target may be made to maintain a desired block error rate (BLER).
With higher data rates in the uplink, a higher chip energy-to-noise ratio (Ec/N0) is needed in order to support the desired throughput. Multi-path propagation in combination with high transmission power (Ec/N0>0 dB) may cause severe self-interference that, in some cases, dominates other interference in the received signal and degrades the overall performance of the mobile terminal. When self-interference is dominant, the received SIR may not be able to reach the SIR target, irrespective of the mobile terminal transmit power because increasing the transmit power also increases the self-interference. In this scenario, inner-loop power control continues to ask the mobile terminal to increase its transmit power, which leads to an undesirable power rush, possible system instability, and serious interference that affects other users' performance.
One possible solution to this problem is to exclude self-interference from the SIR estimation process. For example, an interference suppression receiver, such as a GRAKE receiver, may be used to suppress self-interference when demodulating the received signal. SIR may then be estimated after GRAKE combining. This method has the advantage of being straightforward and the estimated SIR reflects the actual SIR experienced by the modem. However, using an interference suppression receiver to suppress self-interference may not be sufficient to avoid power rushes at high data rates.
Another possible solution is to compute a modified SIR that excludes self-interference and to use the modified SIR for inner-loop power control. The TPC commands are then generated based on the relation between the modified SIR, with self-interference excluded, and the SIR target. However, there is always some residual self-interference that cannot be removed. Further, it may be difficult to accurately estimate the modified SIR at high data rates. Moreover, signal quality is actually affected by self-interference even if the self-interference is discounted when computing the SIR. Removing the effect of self-interference from the SIR estimate results in worse signal quality for a given SIR target and causes the outer-loop power control to compensate for the self-interference.
Another possible solution is to take self-interference into account when determining the data transmission rates for the mobile terminal. When self-interference is dominant, the uplink scheduler may avoid scheduling high data rate transmissions. However, there is an inherent delay in the scheduling process. Consequently, the scheduler cannot respond quickly enough to rapidly changing channel conditions.
U.S. patent application Ser. No. 12/22,346 filed Jan. 30, 2008, titled “Method of Closed Loop Power Control Adjusted by Self-Interference” describes a method of closed loop power control that takes into account the level of self-interference in the received signal when generating power control commands. When the level of self-interference is low, a normal inner-loop power control procedure may be used wherein the signal-to-interference ratio (SIR) of the receive signal is compared to an SIR target to generate power control commands. When the level of self-interference is high, a “fast break” procedure is used for inner-loop power control to constrain further increases in mobile terminal transmit power until the level of self-interference returns to an acceptable level. The “fast break” procedure may reduce the mobile station transmit power, maintain the mobile station transmit power at current levels, or limit further increases in the mobile terminal transmit power. The use of fast break procedures is triggered based on an instantaneous estimate of the orthogonality factor, which can be difficult to compute.